In many applications, it is useful to know the torque of a running motor. One known method of measuring torque involves the use of one or more sensors mechanically coupled to a motor shaft. However, there are certain drawbacks associated with the use of torque measuring sensors. In particular, the sensors need to be built into the rotating equipment which is often mechanically difficult, impractical, or, simply, impossible. In certain applications, the presence of the sensor in the mechanical system can interfere with the motor's operation. For example, in high-speed spindle applications, mechanical sensors can introduce an imbalance into the system (in addition to other issues). Additionally, torque sensors are often expensive.
It is also known to use indirect measurements to determine torque. For example, the “Steinmetz equivalent circuit” may be used to describe how an induction motor's electrical input is transformed into useful mechanical energy output. The parameters of the Steinmetz equivalent circuit can be obtained by starting the motor in a “no load” condition and in a “locked rotor” condition. However, both conditions are disruptive to normal operation of the motor since they each require the motor to be in a non-running state.
Another method of obtaining the Steinmetz equivalent circuit parameters is based on analysis of the motor start-up current by digital signal processing (DSP) analysis. This method involves a learning period for each individual motor and also requires an interruption of the normal operation of the motor system.
U.S. Pat. No. 6,505,132 (Takaku) describes a detection device for detecting instantaneously generated torque and an average generated torque of a three-phase motor. In advance of determining the instantaneously generated torque, the described device is required to detect a “no load” current of the motor.
There therefore exists a need for a method and/or apparatus for estimating the torque of a running motor which does not interfere with the operation of the motor.
It is an object of certain embodiments of the present disclosure to overcome at least some disadvantages associated with the prior art.